I had an issue very similar to this recently, where my 4.7″ iPhones were unable to load the 667h launch image and fell back to the size it could, being the 538h. Since iOS uses the presence of static launch images as one of its determinations for what resolution to display the app at, the app ended up coming out all blown up in resolution.

It doesn’t sound like you’re hitting that latter consequence, but I can share my fix: re-evaluate the color space you used to export those images and try exporting them again. I wasn’t able to pin down the exact reason why the images were an issue for the platform, but re-exporting and cleaning helped. In addition to the color space, did you perhaps use a Save for Web option? I also found that affected things for us.

Have you tried cleaning the project and the build folder, deleting derived data, removing the app from the device and restarting Xcode?

Might sound like a silly suggestion, but this has fixed so many things for me in the past.